Polishing abrasive of crystalline ceric oxide particles having surfaces modified with hydroxyl groups

ABSTRACT

According to the present invention, a process is provided for producing crystalline ceric oxide particles having a particle diameter of 0.005 to 5 mum, which comprises the steps of reacting a cerium (III) salt with an alkaline substance in an (OH)/(Ce3+) molar ratio of 3 to 30 in an aqueous medium in an inert gas atmosphere to produce a suspension of cerium (III) hydroxide, and blowing oxygen or a gas containing oxygen into the suspension at a temperature of 10 to 95° C. and at an atmospheric pressure.

This is a continuation of application Ser. No. 08/899,796 filed Jul. 7,1997 (now U.S. Pat. No. 5,962,343).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing crystallineceric oxide particles. The ceric oxide is used as an abrasive,ultraviolet absorbing material, catalyst material, fuel cell materialand the like. Out of these, the crystalline ceric oxide of the presentinvention provides an excellent material for use as an abrasive and anultraviolet absorbing material.

Further, the present invention relates to the modification of thesurface of a ceric oxide particle or a particle essentially composed ofceric oxide obtained by calcining and grinding or a compositioncontaining a rare earth element essentially composed of cerium, as wellas to an abrasive comprising the surface modified particles and apolishing method.

2. Description of the Related Art

Japanese Laid-open Patent Application No. Sho 63-502656 discloses aprocess for producing ceric oxide particles having a particle diameterof 0.05 to 10 μm by holding an aqueous solution of cerium (III) nitrateor cerium (IV) nitrate in a sealed container at a temperature of 200 to600° C. and at a pressure of not less than 40 atm.

Japanese Laid-open Patent Application No. Hei 6-2582 discloses a processfor producing crystalline ceric oxide particles having a particlediameter of not more than 300 angstroms which comprises cleaning a gelobtained by reacting a cerium salt compound and an alkali metalhydroxide or ammonia to remove impurities, adding an acid such as nitricacid or acetic acid to the gel, and subjecting the resulting mixture toa hydrothermal treatment at 100° C. or more.

Japanese Laid-open Patent Application No. Hei 8-81218 discloses aprocess for producing ceric oxide particles having a particle diameterof 0.03 to 5 μm which comprises adjusting pH values of a solutioncomprising ceric hydroxide and a nitrate to 8 to 11 using an alkalinesubstance and heating the solution at a temperature of 100 to 200° C.under pressurization.

It is known that cerium is an element which is easily oxidized fromvalence (III) to valence (IV) in lanthanoids. For example, it isdescribed on page 348 of “Inorganic Chemistry Vol. 1” written byThunderson and published by Hirokawa Shoten on Apr. 25, 1982 that cerium(IV) is produced when an alkaline suspension of cerium (III) hydroxideis exposed to air.

Since a hydrothermal treatment is carried out at a temperature of notlower than 100° C. under such a condition that a corrosive substancesuch as nitric acid or acetic acid is contained in all the processes ofJapanese Laid-open Patent Application Nos. Sho 63-502656, Hei 6-2582 andHei 8-81218, a high-pressure container that meets the reactionconditions is required and further acid-resistant Teflon, glass or acorrosion-resistant alloy such as hastelloy must be used as a materialfor the high-pressure container.

By the way, it has been proved that cerium oxide particles or theparticles of a composition essentially composed of cerium oxide haveexcellent performance as an abrasive for polishing inorganic glass,quartz crystal and quartz glass.

Japanese Laid-open Patent Application No. Sho 58-55334 discloses aprocess for producing a dispersible product containing a cerium compoundwhich comprises heating a cerium (IV) oxide hydrate in the presence of asalt and disintegrating agglomerated fine crystals contained in thecerium (IV) oxide hydrate. The above publication teaches that ammoniumnitrate is used as the salt in addition to a metal nitrate, metalchloride and metal perchlorate. The publication further teaches that asolution containing a cerium (IV) oxide hydrate and ammonium nitrate wasdried at 105° C. and further heated at 300° C. to obtain a productcontaining ceric oxide and a nitrate which was then dispersed in waterto obtain a gel, as an embodiment of the invention. However, thedescription is not made about the application of this gel.

Japanese Laid-open Patent Application No. Hei 5-262519 discloses aprocess for producing a rare earth oxide which comprises mixing anoxalate, rare earth compound and ammonium salt in an aqueous medium,separating a precipitate produced at 30 to 90° C. and calcining theobtained oxalic acid rare earth ammonium double salt at 600 to 1,200° C.The above publication teaches that ammonium nitrate, ammonium chloride,ammonium acetate or the like is used as the ammonium salt and ceriumnitrate is used as the rare earth compound.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forproducing crystalline ceric oxide from a cerium (III) salt based on areaction mechanism that cerium (IV) is produced when an alkalinesuspension of cerium (III) hydroxide is exposed to an oxidant such asair, by which crystalline ceric oxide particles having a particlediameter controlled to any value within the range of 0.005 to 5 μm areproduced by controlling the nucleus generation and crystal growth speedsof the crystalline ceric oxide particle. As the crystalline ceric oxideparticles are produced at normal pressure (atmospheric pressure) in thisprocess, a bulky high-pressure container is not necessary, therebymaking it possible to produce crystalline ceric oxide particles by safeoperation at a low cost.

A silicon oxide film (SiO₂ film) for a semiconductor device, quartzglass for a photo mask and a quartz piece for a crystal oscillator arebecoming in need of a polished surface having high flatness andaccordingly, an abrasive having a particle diameter in a submicron orlower order must be used.

When the particle diameter of an abrasive used for polishing these isreduced, mechanical polishing force is lowered, thereby lowering thepolishing speed and productivity in the polishing step with the resultof an increase in product cost. If the polishing speed can be increasedwithout changing the particle diameter of an abrasive particle, theproductivity of the polishing step will improve.

When a silicon oxide film (SiO₂ film) is used as a stopper (layer forstopping polishing) for flattening an interlayer film for asemiconductor device, if only a soft film such as an organic resin filmcan be polished without polishing a hard film such as a silicon oxidefilm (SiO₂ film) so that polishing of a soft film such as an organicresin film proceeds while polishing of a hard film such as a siliconoxide film (SiO₂ film) stops, the application of an abrasive tolithography is expected.

It is another object of the present invention to provide an abrasivecomprising ceric oxide particles or particles essentially composed ofceric oxide prepared by heating ceric oxide particles or particlesessentially composed of ceric oxide obtained by calcining and grinding acomposition containing a rare earth element essentially composed ofcerium in an aqueous medium at a temperature of 50 to 250° C. in thepresence of an ammonium salt. This abrasive is composed of surfacemodified ceric oxide particles. The polishing speed can be controlledand a polishing method suitable for the properties of a surface to bepolished can be adopted for an abrasive composed of ceric oxideparticles obtained according to type of a chemical used for surfacemodification.

According to a first aspect of the present invention, there is provideda process for producing crystalline ceric oxide particles having aparticle diameter of 0.005 to 5 μm (micrometer), which comprises thesteps of:

reacting a cerium (III) salt and an alkaline substance in an (OH)/(Ce³⁺)molar ratio of 3 to 30 in an aqueous medium in an inert gas atmosphereto produce a suspension of cerium (III) hydroxide, and

immediately blowing oxygen or a gas containing oxygen into thesuspension at a temperature of 10 to 95° C. and at an atmosphericpressure.

According to a second aspect of the present invention, there is providedan abrasive composed of ceric oxide particles which are heated at atemperature of 50 to 250° C. in an aqueous medium in the presence of anammonium salt.

According to a third aspect of the present invention, there is provideda polishing method using an abrasive composed of ceric oxide particleswhich are heated at a temperature of 50 to 250° C. in an aqueous mediumin the presence of an ammonium salt.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram for comparing the relationship between the lightwavelength and transmittance of glass coated with a ceric oxide solobtained in Example 1 of the present invention and the relationshipbetween the light wavelength and transmittance of glass coated with atitanic oxide sol.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first aspect of the present invention, the first step is to reacta cerium (III) salt with an alkaline substance in an (OH)/(Ce³⁺) molarratio of 3 to 30 in an aqueous medium in an inert gas atmosphere toproduce cerium (III) hydroxide, that is, a suspension of ceroushydroxide.

For the reaction in an inert gas atmosphere, a reactor equipped with agas substitutable stirrer and a thermometer is used to react a cerium(III) salt and an alkaline substance in an aqueous medium. Water isgenerally used as the aqueous medium but a slight amount of awater-soluble organic solvent may be contained in the aqueous medium.Gas substitution is carried out by immersing a tubular gas introductionport into the aqueous medium, blowing an inert gas into the aqueousmedium and discharging the gas from a discharge port provided in theabove of the aqueous medium in the reactor to fill the reactor with theinert gas. It is preferred to start the reaction after the substitutionof the inert gas. Stainless steel, glass lining or the like can be usedas the material of this reactor. At this point, the pressure inside thereactor is desirably an atmospheric pressure and hence, the inflow ofthe gas is preferably almost equal to the outflow of the gas. The inflowand outflow of the gas are preferably 0.01 to 20 liters/min based on 1liter of the reaction tank capacity.

A nitrogen gas, argon gas or the like is used as the inert gas, out ofwhich a nitrogen gas is particularly preferred.

Illustrative examples of the cerium (III) salt include cerous nitrate,cerous chloride, cerous sulfate, cerous carbonate, ammonium cerium (III)nitrate and the like. These cerium (III) salts may be used alone or inadmixture.

Illustrative examples of the alkaline substance include alkali metalhydroxides such as sodium hydroxide and potassium hydroxide and organicbases such as ammonia, amines, quaternary ammonium hydroxide, out ofwhich ammonium, sodium hydroxide and potassium hydroxide areparticularly preferred. They may be used alone or in admixture.

Although the above cerium (III) salt and the alkaline substance may beadded to an aqueous medium to start a reaction in the reactor, anaqueous solution of the cerium (III) salt and an aqueous solution of thealkaline substance are prepared and mixed together to start a reaction.The cerium (III) salt is preferably used in an amount of 1 to 50 wt % inthe aqueous medium.

The ratio of the cerium (III) salt to the alkaline substance is 3 to 30,preferably 6 to 12 in terms of (OH)/(Ce³⁺) molar ratio. When the ratiois smaller than 3, the cerium (III) salt cannot be completelyneutralized into the cerium (III) hydroxide and partially remains in thesuspension disadvantageously. Since this cerium (III) salt is oxidizedinto cerium (IV) much more slowly than cerium (III) hydroxide, when thecerium (III) hydroxide and the cerium (III) salt coexist, the nucleusgeneration speed and the crystal growth speed of the crystalline cericoxide cannot be controlled, resulting in a wide particle sizedistribution and a nonuniform particle diameter disadvantageously. Whenthe (OH)/(Ce³⁺) molar ratio is larger than 30, the crystallinity of theobtained crystalline ceric oxide particles lowers. When the particlesare used as an abrasive, the polishing speed decreasesdisadvantageously. The particle size distribution of the obtainedparticles becomes wide and the particle diameter thereof becomesnonuniform disadvantageously.

The reaction time in the above first step, which differs according tothe amount of charge, is generally 1 minute to 24 hours.

When the cerium (III) salt and the alkaline substance are reacted witheach other in a gas containing oxygen such as air in place of the inertgas in the above first step, the generated cerium (III) hydroxidecontacts oxygen and changes into a cerium (IV) salt and ceric oxide.Therefore, a large number of ceric oxide nuclei are produced in theaqueous medium, resulting in a wide particle size distribution of theobtained ceric oxide particles and a nonuniform particle diameterdisadvantageously.

In the first aspect of the present invention, the second step is toproduce crystalline ceric oxide particles having a particle diameter of0.005 to 5 μm by blowing oxygen or a gas containing oxygen into thesuspension obtained in the first step at a temperature of 10 to 95° C.and at an atmospheric pressure. In other words, the second step is toproduce ceric oxide particles having high crystallinity from the cerium(III) hydroxide in the suspension obtained in the first step through acerium (IV) salt in the presence of oxygen or a gas containing oxygen.The phrase “oxygen or a gas containing oxygen” refers to gaseous oxygen,air or a mixture gas of oxygen and an inert gas. Illustrative examplesof the inert gas include a nitrogen gas, argon gas and the like. When amixture gas is used, the content of oxygen contained in the mixture gasis preferably 1% or more by volume. In the second step of the firstaspect of the present invention, air is particularly preferably usedfrom a viewpoint of ease in production.

The second step of the first aspect of the present invention is carriedout in the same reactor as that of the first step by introducing oxygenor a gas containing oxygen in place of the inert gas introduced in thefirst step immediately after the introduction of the inert gas in thefirst step. That is, oxygen or a gas containing oxygen is blown into thesuspension obtained in the first step from a tubular gas introductionport immersed in the suspension.

Since the second step is carried out at an atmospheric pressure, almostthe same amount of a gas as the amount introduced into the suspension isdischarged from a discharge port provided in the above of the suspensionin the reactor.

In the second step of the first aspect of the present invention, thetotal amount of oxygen or a gas containing oxygen blown into thesuspension is such that the cerium (III) hydroxide can be changed intoceric oxide and the (O₂)/(Ce³⁺) molar ratio is preferably 1 or more. Ifthe molar ratio is less than 1, cerium (III) hydroxide remains in thesuspension. When it contacts oxygen contained in air during cleaningafter the second step, it may produce fine particles with the resultthat the particle size distribution of the obtained ceric oxideparticles becomes wide and the particle diameter thereof becomesnonuniform disadvantageously.

The inflow and outflow of the gas per unit time in the second step ispreferably 0.01 to 50 liters/min based on 1 liter of the reaction tankcapacity.

When the first step of blowing an inert gas and the second step ofblowing oxygen or a gas containing oxygen are not carried outsuccessively, the surface of the suspension obtained in the first stepcontacts air, whereby a layer containing ceric oxide particles havingvarious particle diameters is formed on the surface layer and hence, theparticle diameter of ceric oxide particles obtained in the subsequentsecond step does not become uniform disadvantageously.

The second step of the first aspect of the present invention ispreferably carried out while the suspension is stirred by a stirrer suchas a disperser so that oxygen or a gas containing oxygen is presentuniformly in the suspension. When the suspension is stirred by blowing agas, stirring with a stirrer is not always necessary.

The production of crystalline ceric oxide particles by oxidizing cerium(III) hydroxide in the first aspect of the present invention means thatthe generation of crystalline ceric oxide particle nuclei and the growthof crystals thereof are carried out. The nucleus generation speed andcrystal growth speed can be controlled by the concentration of a ceriumsalt, the concentration of the alkaline substance, the reactiontemperature, and the concentration and supply of an oxidizing aqueoussolution. In the present invention, the concentration of a cerium salt,the concentration of the alkaline substance, the reaction temperature,and the concentration and supply of an oxidizing aqueous solution at thetime of nucleus generation and crystal growth can be changed freely. Theparticle diameter can be controlled to any value within the range of0.005 to 5 μm by adjusting these factors.

The reaction temperature in the second step of the first aspect of thepresent invention greatly contributes to the control of the particlediameter. For instance, when nucleus generation and crystal growth arecarried out at a temperature of 30° C., crystalline ceric oxideparticles having a particle diameter of 5 to 10 nm (nanometer) areobtained and when nucleus generation and crystal growth are carried outat a temperature of 80° C., crystalline ceric oxide particles having aparticle diameter of 80 to 100 nm are obtained. According to the firstaspect of the present invention, there is further provided a secondprocess for obtaining 1 to 3 μm crystalline ceric oxide particles bygrowing crystals using crystalline ceric oxide particles having aparticle diameter of 80 to 100 nm as seed crystals while supplyingcerium (III) hydroxide as a nutriment. That is, in the second process,the second step is carried out successively after the first step byadding crystalline ceric oxide particles having a particle diameter of80 to 100 nm when starting materials are charged in the first step.

In the first aspect of the present invention, when the suspension ofcerium (III) hydroxide containing an oxidizing substance such as nitricacid is subjected to a hydrothermal treatment at a temperature of notlower than 100° C. in an inert gas atmosphere instead of causing thesuspension of cerium (III) hydroxide to react using an oxidant such asair at a temperature of 10 to 95° C. and at normal pressure, onlycrystalline ceric oxide particles having a particle diameter smallerthan a desired value (for example, 30 nm or less) can be obtained. Whenthe suspension of cerium (III) hydroxide containing an oxidizingsubstance such as nitric acid is treated at a temperature of not higherthan 100° C. in an inert gas atmosphere, the reaction is incomplete andan unreacted product remains.

When the suspension of cerium (III) hydroxide containing no oxidizingsubstance is subjected to a hydrothermal treatment at a temperature ofnot lower than 100° C., crystalline ceric oxide particles cannot beobtained.

The ceric oxide particles obtained by the above production process aretaken out from the reactor as a slurry and cleaned by ultrafiltration orfilter press cleaning to remove impurities.

When the ceric oxide particles obtained by the first aspect of thepresent invention are observed by a transmission electron microscope(TEM), they have a particle diameter of 0.005 to 5 μm. When the cericoxide particles are dried at 110° C. and measured for their diffractionpattern by an X-ray diffractometer, they are found to be cubic cericoxide particles having main peaks at diffraction angles 2θ of 28.6°,47.5° and 56.4° and high crystallinity as specified in ASTM Card No.34-394. The specific surface area value measured by a gas adsorptionmethod (BET method) of the ceric oxide particles is 2 to 200 m²/g.

The crystalline ceric oxide particles obtained by the first aspect ofthe present invention can be used as an abrasive, ultraviolet absorbingmaterial, catalyst material, fuel cell material and the like as a cericoxide sol which is prepared by re-dispersing the particles in a watermedium, water-soluble organic solvent or a mixture solvent of water anda water-soluble organic solvent. In addition, the above ceric oxide solcan increase the concentration of ceric oxide up to 60 wt %. When theceric oxide sol obtained by the first aspect of the present invention isleft for a long time, part of the particles precipitates but can bere-dispersed easily by stirring and return to a dispersion state at thetime of production. Therefore, the ceric oxide sol is stable for morethan one year when it is kept at normal temperature.

When the ceric oxide sol of the first aspect of the present invention isused as an abrasive, it is an excellent abrasive for final finishingbecause it can greatly improve in the surface properties of the obtainedpolished surface with reduced surface roughness as compared with anabrasive composed of a ceric oxide powder slurry produced by calciningand grinding. In addition, the surface properties of a surface polishedby the abrasive of the present invention are as good as and thepolishing speed is several times faster than an abrasive composed of aconventional ceric oxide sol produced by a hydrothermal treatment. Thereason for this seems to be that the surface of each particle ischemically active because the production temperature of the ceric oxideparticle of the first aspect of the present invention is low.

Therefore, when the ceric oxide sol obtained by the first aspect of thepresent invention is used, the time required for the polishing step canbe reduced and a high-quality polished surface can be obtained.

When quaternary ammonium ions (NR₄ ⁺ wherein R is an organic group) arecontained in the ceric oxide sol obtained by the first aspect of thepresent invention in a (NR₄ ⁺)/CeO₂) molar ratio of 0.001 to 1 afterimpurities are removed by cleaning, the stability of an abrasivesolution is improved advantageously. The quaternary ammonium ions areprovided by adding quaternary ammonium silicate, quaternary ammoniumhalide, quaternary ammonium hydroxide or a mixture thereof, out of whichquaternary ammonium silicate and quaternary ammonium hydroxide areparticularly preferred. R is a methyl group, ethyl group, propyl group,hydroxyethyl group, benzyl group or the like. Illustrative examples ofthe quaternary ammonium compound include tetramethylammonium silicate,tetraethylammonium silicate, tetraethanolammonium silicate, monoethyltriethanolammonium silicate, trimethylbenzylammonium silicate,tetramethylammonium hydroxide and tetraethylammonium hydroxide.

A trace amount of an acid or a base can be contained. The above abrasivesolution (sol) can be changed into an acidic abrasive solution (sol) byadding a water-soluble acid in a [H⁺]/[CeO₂] molar ratio of 0.001 to 1.This acidic sol has a pH of 1 to 6, preferably 2 to 6. Illustrativeexamples of the water-soluble acid include inorganic acids such ashydrogen chloride and nitric acid, organic acids such as formic acid,acetic acid, oxalic acid, tartaric acid, citric acid and lactic acid,acidic salts thereof and mixtures thereof. When a water-soluble base iscontained in an [OH⁻]/[CeO₂] molar ratio of 0.001 to 1, the abrasivesolution can be changed into an alkaline sol. This alkaline abrasivesolution (sol) has a pH of 8 to 13, preferably 9 to 13. Illustrativeexamples of the water-soluble base include amines such asmonoethanolamine, diethanolamine, triethanolamine,aminoethylethanolamine, N,N-dimethylethanolamine, N-methylethanolamine,monopropanolamine and morpholine, and ammonia in addition to theabove-described quaternary ammonium silicate and quaternary ammoniumhydroxide.

The ceric oxide sol obtained by the first aspect of the presentinvention is superior as an ultraviolet absorber in ultravioletabsorption property to titanic oxide.

According to a second aspect of the present invention, there is providedan abrasive composed of ceric oxide particles which are heated at atemperature of 50 to 250° C. in an aqueous medium in the presence of anammonium salt.

Further, according to a third aspect of the present invention, there isprovided a polishing method using an abrasive composed of ceric oxideparticles which are heated at a temperature of 50 to 250° C. in anaqueous medium in the presence of an ammonium salt.

In the second and third aspects of the present invention, the cericoxide particles used as a starting material are not limited to aparticular kind, and ceric oxide particles produced by known methods canbe used. For example, it is possible to use crystalline ceric oxideparticles having a particle diameter of 0.05 to 5 μm produced by a dryprocess which comprises calcining a cerium salt such as ceroushydroxide, ceric hydroxide, cerous nitrate, basic cerium (IV) nitrate,cerous chloride, ceric chloride, cerous carbonate, ceric carbonate,basic cerous sulfate, basic ceric sulfate, cerous oxalate or cericoxalate, or a rare earth compound essentially composed thereof at 1,000°C., grinding and classifying.

The crystalline ceric oxide particles obtained by the first aspect ofthe present invention are particularly preferably used as the cericoxide particles that are used as a starting material in the second andthird aspects of the present invention.

The aqueous medium of the second and third aspects of the presentinvention is generally water but can contain a trace amount of awater-soluble organic medium.

The ammonium salt used in the second and third aspects of the presentinvention may be an ammonium salt having a non-oxidizing anioncomponent. This ammonium salt having a non-oxidizing anion component isthe most preferably ammonium carbonate or ammonium hydrogencarbonate.They may be used alone or in admixture.

The ammonium salt having a non-oxidizing anion component is preferablycontained in an aqueous medium in an [NH₄ ⁺]/[CeO₂] molar ratio of 0.1to 30 and the concentration of the ammonium salt in the aqueous mediumis preferably 1 to 30 wt %.

When the ceric oxide particles are heated in an aqueous medium using theammonium salt having a non-oxidizing anion component, they are heated ata temperature of 50 to 250° C., preferably 50 to 180° C. to obtainsurface modified crystalline ceric oxide particles. The heating time canbe set to 10 minutes to 48 hours. When the heating temperature is 100°C. or lower, an open (not-sealed) reactor is used. When the heatingtemperature is higher than 100° C., an autoclave or supercriticaltreatment apparatus is used. The heated ceric oxide particles are takenout from a treatment layer as a slurry and cleaned by ultrafiltration orfilter press to remove impurities.

The ceric oxide particles whose surfaces are modified by heating in thepresence of an ammonium salt having a non-oxidizing anion component canbe easily dispersed into an aqueous medium to prepare an abrasivesolution. This aqueous medium is preferably water.

When quaternary ammonium ions (NR₄ ⁺ wherein R is an organic group) arecontained in the sol of the ceric oxide particles whose surfaces aremodified by heating in an aqueous medium in the presence of an ammoniumsalt having a non-oxidizing anion component in a (NR₄ ⁺)/(CeO₂) molarratio of 0.001 to 1 after impurities are removed from the particles bycleaning, the stability of the abrasive solution is improvedadvantageously. The quaternary ammonium ions are provided by addingquaternary ammonium silicate, quaternary ammonium halide, quaternaryammonium hydroxide or a mixture thereof, out of which quaternaryammonium silicate and quaternary ammonium hydroxide are particularlypreferred. R is a methyl group, ethyl group, propyl group, hydroxyethylgroup, benzyl group or the like. Illustrative examples of the quaternaryammonium compound include tetramethylammonium silicate,tetraethylammonium silicate, tetraethanolammonium silicate, monoethyltriethanolammonium silicate, trimethylbenzylammonium silicate,tetramethylammonium hydroxide and tetraethylammonium hydroxide.

A trace amount of an acid or a base can be contained. The pH of theabrasive solution is preferably 2 to 12. The above abrasive solution(sol) can be changed into an acidic abrasive solution (sol) by adding awater-soluble acid in a [H⁺]/[CeO₂] molar ratio of 0.001 to 1. Thisacidic sol has a pH of 2 to 6. Illustrative examples of thewater-soluble acid include inorganic acids such as hydrogen chloride andnitric acid, organic acids such as formic acid, acetic acid, oxalicacid, tartaric acid, citric acid and lactic acid, acidic salts thereofand mixtures thereof. When a water-soluble base is contained in an[OH⁻]/[CeO₂] molar ratio of 0.001 to 1, the abrasive solution can bechanged into an alkaline sol. This alkaline abrasive solution (sol) hasa pH of 8 to 12. Illustrative examples of the water-soluble base includeamines such as monoethanolamine, diethanolamine, triethanolamine,aminoethylethanolamine, N,N-dimethylethanolamine, N-methylethanolamine,monopropanolamine and morpholine, and ammonia in addition to theabove-described quaternary ammonium silicate and quaternary ammoniumhydroxide.

The abrasive solution is stable for more than 1 year when it is kept atroom temperature.

A polishing method using an abrasive containing ceric oxide whosesurface is modified by heating in the presence of the above ammoniumsalt having a non-oxidizing anion component makes it possible to achievea higher speed of polishing a silicon oxide film (SiO₂ film) for asemiconductor device, inorganic glass, crystal (such as a quartz piecefor a crystal oscillator) and quartz glass than a polishing method usingceric oxide particles having the same particle diameter of the priorart. The reason for this is that the ceric oxide particles have both amechanical polishing function and a chemical polishing function, a largenumber of hydroxyl groups (≡Ce—OH) are produced on the surface of theceric oxide particle by heating in an aqueous medium in the presence ofan ammonium salt having a non-oxidizing anion component, and this group(≡Ce—OH) exerts a chemical effect on the hydroxyl group (≡Si—OH) on thesurface of a silicon oxide film, thereby improving the polishing speed.The ammonium salt having a non-oxidizing anion component is consideredto exert a reducing effect on the surface of the ceric oxide particle.

In the second and third aspects of the present invention, the aboveeffect (high polishing speed can be obtained) can be also obtained byusing, as an abrasive, ceric oxide particles which have been heated inan aqueous medium in the presence of a water-soluble reducing substancesuch as hydrazine, ammonium nitrite or the like instead of heating in anaqueous medium in the presence of above ammonium salt having anon-oxidizing anion component. However, the above ammonium salt having anon-oxidizing anion component is preferably used because hydrazine andammonium nitrite involve a risk such as explosion during handling.

Even when the ceric oxide particles are heated in purified water withoutusing the above ammonium salt having a non-oxidizing anion component,the polishing speed is the same as in the case where starting materialceric oxide particles are used as an abrasive and hence, the aboveeffect cannot be expected.

It is known that the polishing speed slows down gradually when anabrasive is used in the polishing step. Since an abrasive having areduced polishing speed (that is, reduced polishing ability) causes areduction in productivity in the polishing step, the abrasive itself hasto be disposed. However, in the second and third aspects of the presentinvention, the used ceric oxide particles having reduced polishingability can restore its original polishing ability and achieve improvedpolishing speed by heating at a temperature of 50 to 250° C. in anaqueous medium in the presence of an ammonium salt having anon-oxidizing anion component.

On the other hand, the ammonium salt used in the second and thirdaspects of the present invention can be an ammonium salt having anoxidizing anion component. This ammonium salt having an oxidizing anioncomponent is the most preferably ammonium nitrate or ammonium sulfamate.They may be used alone or in admixture.

The above ammonium salt having an oxidizing anion component ispreferably contained in an aqueous medium in an [NH₄ ⁺]/[CeO₂] molarratio of 0.1 to 30 and in a concentration of 1 to 30 wt %.

When the ceric oxide particles are heated in an aqueous medium using anammonium salt having an oxidizing anion component, they are heated at 50to 250° C., preferably 100 to 250° C. to obtain surface modified cericoxide particles. The heating time can be set to 10 minutes to 96 hours.When the heating temperature is 100° C. or lower, an open (not-sealed)reactor is used. When the heating temperature is higher than 100° C., anautoclave or supercritical treatment apparatus is used.

The heated ceric oxide particles are taken out from a treatment layer asa slurry and cleaned by ultrafiltration or filter press to removeimpurities.

The ceric oxide particles whose surfaces are modified by heating in thepresence of an ammonium salt having an oxidizing anion component can beeasily dispersed in an aqueous medium to prepare an abrasion solution.As the aqueous medium may be used water.

When quaternary ammonium ions (NR₄ ⁺ wherein R is an organic group) arecontained in the sol of the ceric oxide particles whose surfaces aremodified by heating in an aqueous medium in the presence of an ammoniumsalt having an oxidizing anion component in a (NR₄ ⁺)/(CeO₂) molar ratioof 0.001 to 1 after impurities are removed by cleaning, the stability ofthe obtained abrasive solution is improved and the above effect (only asoft film such as an organic resin film is polished without polishing ahard film such as a silicon oxide film) is further improvedadvantageously. The quaternary ammonium ions are provided by addingquaternary ammonium silicate, quaternary ammonium halide, quaternaryammonium hydroxide or a mixture thereof, out of which quaternaryammonium silicate and quaternary ammonium hydroxide are particularlypreferred. R is a methyl group, ethyl group, propyl group, hydroxyethylgroup, benzyl group or the like. Illustrative examples of the quaternaryammonium compound include tetramethylammonium silicate,tetraethylammonium silicate, tetraethanolammonium silicate, monoethyltriethanolammonium silicate, trimethylbenzylammonium silicate,tetramethylammonium hydroxide and tetraethylammonium hydroxide.

A trace amount of a base can be contained. The pH of the abrasivesolution is preferably 8 to 12. When a water-soluble base is containedin an [OH⁻]/[CeO₂] molar ratio of 0.001 to 1, the abrasive solution(sol) can be changed into an alkaline sol. This alkaline abrasivesolution (sol) has a pH of 8 to 12. Illustrative examples of thewater-soluble base include amines such as monoethanolamine,diethanolamine, triethanolamine, aminoethylethanolamine,N,N-dimethylethanolamine, N-methylethanolamine, monopropanolamine andmorpholine, and ammonia in addition to the above-described quaternaryammonium silicate and quaternary ammonium hydroxide.

The abrasive solution is stable for more than 1 year when it is kept atroom temperature.

A polishing method using the surface modified ceric oxide particleswhich have been heated in the presence of an ammonium salt having anoxidizing anion component can be used as an abrasive to flatten aninterlayer film for a semiconductor device. For instance, when a siliconoxide film (SiO₂ film) serves as a stopper (layer for stoppingpolishing), if only a soft film such as an organic resin film can bepolished without polishing a hard film such as a silicon oxide film(SiO₂ film), that is, polishing of a soft layer such as an organic resinlayer proceeds while polishing of a hard film such as a silicon oxidefilm (SiO₂ film) stops, lithography is made possible. That is, theabrasive has a low polishing speed for a silicon oxide film (SiO₂ film).The reason for this is assumed to be that the ceric oxide particles haveboth a mechanical polishing function and a chemical polishing functionand hydroxyl groups (≡Ce—OH) on the surface of the ceric oxide particleexerts a chemical effect on hydroxyl groups (≡Si—OH) on the surface of asilicon oxide film, thereby improving the polishing speed. It isconsidered that by heating in an aqueous medium in the presence of anammonium salt having an oxidizing anion component, the number of thehydroxyl groups (≡Ce—OH) on the surface of the ceric oxide particle isreduced and the chemical polishing function of the ceric oxide particleis lowered. The ammonium salt having an oxidizing anion component isassumed to exert an oxidizing effect on the surface of the ceric oxideparticle.

For example, in the planarization of an interlayer film for asemiconductor device, the polishing speed can be freely controlled byusing an abrasive containing ceric oxide particles heated in an aqueousmedium in the presence of an ammonium salt having a non-oxidizing anioncomponent and an abrasive containing ceric oxide particles heated in anaqueous medium in the presence of an ammonium salt having an oxidizinganion component alternately for polishing. With this polishing method, amaterial to be polished including a hard silicon oxide film (SiO₂)portion and a soft organic resin film portion can be polished with highaccuracy using two different abrasives. The word “alternately” meansthat each of these abrasives is used separately to polish a surface tobe polished at least one time in this method.

In the second and third aspects of the present invention, when the cericoxide particles which have been heated at a temperature of 50 to 250° C.in an aqueous medium in the presence-of an ammonium salt having anon-oxidizing anion component or an ammonium salt having an oxidizinganion component are dried and measured for their diffraction pattern byan X-ray diffractometer, they are found to be cubic ceric oxideparticles having main peaks at diffraction angles 2θ of 28.6°, 47.5° and56.4° and high crystallinity as specified in ASTM Card No. 34-394.

EXAMPLE 1

37.0 kg of a 9 wt % ammonium aqueous solution which was equivalent to anNH₃/Ce³⁺ molar ratio of 6 was charged into a 100-liter stainless steelreaction tank and a nitrogen gas was blown into the reaction tank from aglass nozzle at a rate of 2 Nm³/hr to substitute the inside of thereaction tank with the nitrogen gas while the temperature of thesolution was maintained at 30° C. A solution of 14.0 kg of cerium (III)nitrate dissolved in 40 kg of purified water was added to the reactiontank little by little to obtain a suspension of cerium (III) hydroxide.After this suspension was heated to 80° C., air was blown into the tankfrom the glass nozzle at a rate of 4 Nm³/hr in place of the nitrogen gasto start an oxidation reaction for converting cerium (III) into cerium(IV). The oxidation reaction ended in 3 hours. When the solution afterthe reaction was returned to room temperature, a reaction solutionhaving light yellow fine particles and a pH of 7.2 was obtained.

When the fine particles were separated from the reaction solution byfiltration, cleaned and observed through a transmission electronmicroscope (TEM), they were found to be particles having a particlediameter of 80 to 100 nm. The yield of the particles was almost 100%.When the fine particles were dried and measured for their powder X-raydiffraction, they had main peaks at diffraction angles 20 of 28.6°,47.5° and 56.4° which coincided with the characteristic peaks of cubiccrystalline ceric oxide specified in ASTM Card No. 34-394. The specificsurface area value measured by a gas adsorption method (BET method) ofthe ceric oxide particles was 30 m²/g.

A 10 wt % nitric acid aqueous solution was added to cleaned CeO₂particles in a [HNO₃/CeO₂] molar ratio of 0.01 to adjust theconcentration of CeO₂ to 5 wt % and pH to 5.0. A polishing test wascarried out using this sol as an abrasive solution under the followingconditions.

The abrasive of Example 1 was polished by a commercially availablecorrection ring type polishing machine. The polishing speed was obtainedby forming a groove in a crystal base and measuring the depth of thegroove with a tracer type surface roughness meter.

a. polishing cloth: polyurethane impregnated nonwoven cloth (φ250 mm)

b. object to be polished: AT cut quartz crystal base (φ14×1.0 mm)

c. revolution: 70 rpm

d. polishing pressure: 400 g/cm², and

e. polishing time: 60 minutes

Table 1 shows polishing characteristics (polishing speed and surfaceroughness Rmax value)

A sol having a CeO₂ concentration of 5 wt % was applied to quartz glassto a film thickness of 0.5 μm by an applicator having a 0.01 mmclearance. The sol was dried at 110° C. for 1 hour and used for themeasurement of transmittance of light having a wavelength range of 200to 600 nm using an ultraviolet absorption meter to measure theabsorption of light having an ultraviolet to visible range of thecoating glass. The result is shown in FIG. 1. For comparison, acommercially available titanium oxide sol was coated on quartz glass andused for the measurement of transmittance of light having an ultravioletto visible range. The result is shown in FIG. 1. In FIG. 1, the axis ofabscissa indicates the wavelength of light (nm) and the axis of ordinateindicates transmittance (%). As the lower the transmittance, the higherabsorption power a sample has.

EXAMPLE 2

740 g of a 9 wt % ammonium aqueous solution which was equivalent to anNH₃/Ce³⁺ molar ratio of 6 was charged into a 2-liter separable flask anda nitrogen gas was blown into the flask from a glass nozzle at a rate of2 liters/min to substitute the inside of the flask with the nitrogen gaswhile the temperature of the solution was maintained at 30° C. Asolution of 76.8 g of cerium (III) nitrate dissolved in 500 g ofpurified water was added to the flask little by little to obtain asuspension of cerium (III) hydroxide. Thereafter, air was blown into theflask from the glass nozzle at a rate of 4 liters/min in place of thenitrogen gas to start an oxidation reaction for converting cerium (III)into cerium (IV) while the temperature of the suspension was maintainedat 30° C. The oxidation reaction ended in 12 hours. When the solutionafter the reaction was returned to room temperature, a reaction solutionhaving yellow fine particles and a pH of 6.5 was obtained.

When the fine particles were separated from the reaction solution byfiltration, cleaned and observed through a transmission electronmicroscope (TEM), they were found to be particles having a particlediameter of 5 to 10 nm. When the particles were dried and measured fortheir powder X-ray diffraction, their main peaks coincided with thecharacteristic peaks of cubic crystalline ceric oxide.

The specific surface area value measured by a gas adsorption method (BETmethod) of the ceric oxide particles was 94 m²/g.

EXAMPLE 3

740 g of a 21 wt % sodium hydroxide aqueous solution which wasequivalent to a NaOH/Ce³⁺ molar ratio of 6 was charged into a 2-literseparable flask and a nitrogen gas was blown into the flask from a glassnozzle at a rate of 2 liters/min to substitute the inside of the flaskwith the nitrogen gas while the temperature of the solution wasmaintained at 30° C. A solution of 216 g of cerium (III) nitratedissolved in 500 g of purified water was added to the flask little bylittle to obtain a suspension of cerium (III) hydroxide. After thesuspension was heated to 80° C., air was blown from the glass nozzle ata rate of 4 liters/min in place of the nitrogen gas to start anoxidation reaction for converting cerium (III) into cerium (IV). Theoxidation reaction ended in 12 hours. When the solution after thereaction was returned to room temperature, a reaction solution havinglight yellow fine particles and a pH of 8.5 was obtained.

When the fine particles were separated from the reaction solution byfiltration, cleaned and observed through a transmission electronmicroscope (TEM), they were found to be particles having a particlediameter of 10 to 20 nm. When the particles were dried and measured fortheir powder X-ray diffraction, their main peaks coincided with thecharacteristic peaks of cubic crystalline ceric oxide.

COMPARATIVE EXAMPLE 1

433 g of cerium (III) nitrate and 2,000 g of purified water were chargedinto a beaker and 60 g of 28 wt % ammonium water was added to the beakerin an NH₃/Ce⁴⁺ molar ratio of 1 under agitation to obtain a slurrycontaining a colloidal precipitate of cerium (IV) hydroxide. This slurrywas placed into a 3-liter autoclave apparatus made from glass lining andsubjected to a hydrothermal treatment at a temperature of 150° C. and apressure of 4 kg/cm² for 20 hours. When the solution after the reactionwas returned to room temperature and atmospheric pressure, a reactionsolution having light yellow fine particles and a pH of 1.4 wasobtained.

When the fine particles were separated from the reaction solution byfiltration, cleaned and analyzed as in Example 1, they were found to becrystalline ceric oxide particles having a particle diameter of 15 to 25nm.

The specific surface area measured by a gas adsorption method (BETmethod) of the ceric oxide particles was 54 m²/g. A 10 wt % nitric acidaqueous solution was added to the cleaned CeO₂ particles in a[HNO₃/CeO₂] molar ratio of 0.01 to adjust the concentration of CeO₂ to 5wt % and pH to 5.0. The polishing characteristics (polishing speed andsurface roughness Rmax value) of this sol as an abrasive were evaluatedunder the same conditions as in Example 1 and the results are shown inTable 1.

As a reference, commercially available ceria powders (composed of cericoxide having an average particle diameter of 0.7 μm) were dispersed inpurified water and a 10 wt % nitric acid aqueous solution was added tothe dispersion in an [HNO₃/CeO₂] molar ratio of 0.01 to adjust theconcentration of CeO₂ to 5 wt % and pH to 5.0. The polishingcharacteristics (polishing speed and surface roughness Rmax value) ofthis slurry as an abrasive were evaluated under the same conditions asin Example 1 and the results are shown in Table 1.

COMPARATIVE EXAMPLE 2

43 g of cerium (III) nitrate and 200 g of purified water were chargedinto a beaker and heated under agitation until they were boiled, and 29g of a 35 wt % hydrogen peroxide solution was added to the mixturelittle by little without causing the bumping of the solution to start anoxidation reaction for converting cerium (III) into cerium (IV). Thethus obtained aqueous solution of basic cerium (IV) nitrate was cooled,and 24 g of a 28 wt % ammonium solution which was equivalent to anNH₃/Ce⁴⁺ molar ratio of 4 was added to the aqueous solution underagitation to obtain a slurry containing ammonium nitrate having anNO₃/Ce⁴⁺ molar ratio of 3, ammonia having an NH₃/Ce⁴⁺ molar ratio of 1and a colloidal precipitate of cerium (IV) hydroxide. After thiscolloidal precipitate was separated, cleaned with purified water andre-dispersed, its pH was adjusted to 5.0 with dilute nitric acid. 85 gof this slurry was charged into a 120 cc Teflon autoclave apparatus andsubjected to a hydrothermal treatment at a temperature of 180° C. and apressure of 10 kg/cm² for 15 hours. When the solution after the reactionwas returned to room temperature and atmospheric pressure, a reactionsolution having light yellow fine particles and a pH of 0.8 wasobtained. When the fine particles were separated from the reactionsolution by filtration, cleaned and analyzed as in Example 1, they werefound to be crystalline ceric oxide particles having a particle diameterof 5 to 10 nm.

TABLE 1 polishing characteristics ceric oxide polishing speed surfaceroughness abrasive (μm/hr) Rmax (angstrom) Example 1 3.2 30 Comparative1.3 30 Example 1 commercially-available 4.6 65 ceria powders

Table 1 shows that an abrasive composed of the ceric oxide particles ofExample 1 obtained by the process of the present invention has apolishing speed about 2.5 times higher than an abrasive composed of theceric oxide particles of Comparative Example 1 obtained by ahydrothermal method. The surface polished by the abrasive composed ofthe ceric oxide particles of Example 1 obtained by the process of thepresent invention is more smooth than the surface polished by anabrasive composed of commercially available ceric oxide particlesobtained by a calcining method.

When the ceric oxide sol obtained in Example 1 or Comparative Example 1is used as an abrasive, the surface flatness remains unchanged even whenthe polished surface is etched because processing distortion is small.However, when a slurry obtained by dispersing commercially availableceria powders used as a reference in purified water is used as anabrasive, the polished surface becomes uneven by etchingdisadvantageously because processing distortion is large.

It is understood from FIG. 1 that the ceric oxide of the presentinvention has higher ultraviolet absorption power than titanic oxide.

EXAMPLE 4

740 g of a 9 wt % ammonium aqueous solution which was equivalent to anNH₄OH/Ce³⁺ molar ratio of 6 was charged into a 2-liter separable flaskand a nitrogen gas was blown into the flask from a glass nozzle at arate of 2 liters/min to substitute the inside of the flask with thenitrogen gas while the temperature of the solution was maintained at 30°C. A solution of 216 g of cerium (III) nitrate dissolved in 500 g ofpurified water was added to the flask little by little to obtain asuspension of cerium (III) hydroxide. After the suspension was heated to80° C., air was blown from the glass nozzle at a rate of 4 liters/min inplace of the nitrogen gas to start an oxidation reaction for convertingcerium (III) into cerium (IV). The oxidation reaction ended in 3 hoursand a reaction solution having light yellow ceric oxide particles and apH of 7.2 was obtained. The ceric oxide particles were used as startingmaterial particles.

When the ceric oxide particles which would be the starting materialparticles were separated from the reaction solution by filtration,cleaned and observed through a transmission electron microscope (TEM),they were found to be particles having a particle diameter of 80 to 100nm. When the particles were dried and measured for their powder X-raydiffraction, they had major peaks at diffraction angles 2θ of 28.6°,47.5° and 56.4° which coincided with the characteristic peaks of cubiccrystalline ceric oxide specified in ASTM Card No. 34-394.

1,500 g of a 10 wt % ammonium carbonate aqueous solution was chargedinto a 3-liter separable flask and 150 g of the ceric oxide particles asstarting material particles obtained above were added to the flask andheated at 95° C. for 8 hours. When the heated slurry was separated byfiltration, cleaned and observed through a transmission electronmicroscope (TEM), they were found to have the same particle diameter of80 to 100 nm as that before surface modification. When the surfacemodified particles were dried and measured for their powder X-raydiffraction, they were found to be cubic ceric oxide particles. Atetramethyl ammonium silicate aqueous solution was added to the surfacemodified crystalline ceric oxide particles which were filtered andcleaned again in an [N(CH₃)₄ ⁺/CeO₂] molar ratio of 0.01 to adjust theconcentration of CeO₂ to 20 wt % and pH to 10.3. 750 g of this 20 wt %crystalline ceric oxide sol was prepared as an abrasive solution.

EXAMPLE 5

1,500 g of a 10 wt % ammonium carbonate aqueous solution was chargedinto a 3-liter high-pressure container made from glass lining and 150 gof ceric oxide particles which were produced in the same manner as inExample 4 as starting material particles were placed in thehigh-pressure container and subjected to a hydrothermal treatment at150° C. for 20 hours. When the heated slurry was separated byfiltration, cleaned and observed through a transmission electronmicroscope (TEM), they were found to have the same particle diameter of80 to 100 nm as that before surface modification. When the surfacemodified particles were dried and measured for their powder X-raydiffraction, they were found to be cubic ceric oxide particles. Atetramethyl ammonium silicate aqueous solution was added to the surfacemodified crystalline ceric oxide particles which were filtered andcleaned again in an [N(CH₃)₄ ⁺/CeO₂] molar ratio of 0.01 to adjust theconcentration of CeO₂ to 20 wt % and pH to 10.3. 750 g of this 20 wt %crystalline ceric oxide sol was prepared as an abrasive solution.

EXAMPLE 6

1,500 g of a 10 wt % ammonium nitrate aqueous solution was charged intoa 3-liter high-pressure container made from glass lining and 150 g ofceric oxide particles which were produced in the same manner as inExample 4 as starting material particles were placed in thehigh-pressure container and subjected to a hydrothermal treatment at150° C. for 20 hours. When the heated slurry was separated byfiltration, cleaned and observed through a transmission electronmicroscope (TEM), they were found to have the same particle diameter of80 to 100 nm as that before surface modification. When the surfacemodified particles were dried and measured for their powder X-raydiffraction, they were found to be cubic ceric oxide particles. Thesurface modified crystalline ceric oxide particles which were filteredand cleaned again were dispersed in purified water and further atetramethyl ammonium hydroxide aqueous solution was added to thedispersion in an [N(CH₃)₄ ⁺/CeO₂] molar ratio of 0.01 to stabilize pH at10.3. 750 g of this 20 wt % crystalline ceric oxide sol was prepared asan abrasive solution.

EXAMPLE 7

1,500 g of a 10 wt % ammonium carbonate aqueous solution was chargedinto a 3-liter separable flask and 150 g of abrasive particles (particlediameter of 1 μm, CeO₂ content of 88 wt %, La₂O₃ content of 7 wt % andSiO₂ content of 5 wt %) obtained by calcining and grinding acommercially available rare earth compound essentially composed ofcerium were placed in the separable flask and heated at 95° C. for 8hours to modify the surface of each particle. When the heated slurry wasseparated by filtration, cleaned and observed through a transmissionelectron microscope (TEM), they were found to have the same particlediameter of 1 μm as that before surface modification. When the obtainedsurface modified particles were dried and measured for their powderX-ray diffraction, it was found that their main peaks included the peaksof cubic ceric oxide. A tetramethyl ammonium silicate aqueous solutionwas added to the above surface modified crystalline ceric oxideparticles which were filtered and cleaned again in an [N(CH₃)₄ ⁺/CeO₂]molar ratio of 0.01 to adjust the concentration of CeO₂ to 20 wt % andpH to 10.3. 750 g of this 20 wt % abrasive particle dispersion wasprepared as an abrasive solution.

COMPARATIVE EXAMPLE 3

Ceric oxide particles having a particle diameter of 80 to 100 nm whichwere produced in the same manner as in Example 4 as starting materialparticles were dispersed in purified water without being surfacemodified by heating in the presence of an ammonium salt and atetramethyl ammonium silicate aqueous solution was added to thedispersion in an [N(CH₃)₄ ⁺/CeO₂] molar ratio of 0.01 to adjust theconcentration of CeO₂ to 20 wt % and pH to 10.3. 750 g of this 20 wt %crystalline ceric oxide sol was prepared as an abrasive solution.

COMPARATIVE EXAMPLE 4

Abrasive particles (particle diameter of 1 μm, CeO₂ content of 88 wt %,La₂O₃ content of 7 wt % and SiO₂ content of 5 wt %) obtained bycalcining and grinding a commercially available rare earth compoundessentially composed of cerium were dispersed in purified water withoutsurface modification by heating in the presence of an ammonium salt anda tetramethyl ammonium silicate aqueous solution was added to thedispersion in an [N(CH₃)₄ ⁺/CeO₂] molar ratio of 0.01 to adjust theconcentration of CeO₂ to 20 wt % and pH to 10.3. 750 g of this 20 wt %crystalline ceric oxide sol was prepared as an abrasive solution.

The abrasives of Examples 4 to 7 and Comparative Examples 3 and 4 werepolished under the following conditions using a commercially availableOscar type lens polishing machine. The polishing speed was obtained byforming a groove in crystal glass and measuring the depth of the groovewith a tracer type surface roughness meter. The results are shown inTable 2.

a. polishing cloth: fluororesin (φ250 mm) or foamed polyurethane (φ250mm)

b. object to be polished: quartz glass (φ25 mm)

c. revolution: 30 rpm, and

d. polishing pressure: 270 g/cm²

TABLE 2 polishing speed (nm/min) fluororesin as foamed polyurethaneabrasive polishing cloth as polishing cloth Example 4 46 90 Example 5 4083 Example 6 1 — Example 7 — 502 Comparative 21 38 Example 3 Comparative— 438 Example 4

The abrasives of Examples 4 and 5 composed of ceric oxide particleswhose surfaces were modified by heating in an aqueous medium in thepresence of an ammonium salt having a non-oxidizing anion component havea polishing speed almost double that of the abrasive of ComparativeExample 3 composed of ceric oxide particles whose surfaces were notmodified under the same polishing conditions. Abrasives having animproved polishing speed as shown by Examples 4 and 5 can achieve ahigher polishing speed than an abrasive composed of ceric oxideparticles of the prior art having the same particle diameter as that ofthe above abrasives for the polishing of a hard film such as a siliconoxide film (SiO₂ film) of a semiconductor device and a hard substancesuch as inorganic glass, crystal (such as a quartz piece of a crystaloscillator) or quartz glass.

As for the relationship between the abrasives of Examples 4 and 5 andthe abrasive of Comparative Example 3, an abrasive composed of surfacemodified particles has a high polishing speed as in the relationshipbetween the abrasive of Example 7 and the abrasive of ComparativeExample 4 even when abrasive particles obtained by calcining andgrinding a commercially available rare earth compound essentiallycomposed of cerium are used as starting material particles.

On the other hand, the abrasive of Example 6 composed of ceric oxideparticles whose surfaces are modified by heating in an aqueous medium inthe presence of an ammonium salt having an oxidizing anion component hasa polishing speed 1/20 that of the abrasive of Comparative Example 3composed of ceric oxide particles whose surfaces are not been modified,under the same polishing conditions.

As for usage of the abrasive of Example 6, supposing that the abrasiveof Example 6 of the present invention is used to polish a hard SiO₂ filmformed on the uneven surface of an LSI chip as a stopper and a soft film(an organic resin film or SiO₂ film containing impurities) formed on thehard SiO₂ film to flatten an interlayer film using CMP, for example, thesoft film formed on the surfaces of projecting portions is firstpolished and then polishing is stopped at hard SiO₂ film portionsexposed to the projecting portions, whereby only soft film portions canbe polished selectively.

Supposing that the abrasives of Examples 4 and 5 and the abrasive ofExample 6 are combined to polish a soft film (an organic resin film orSiO₂ film containing impurities) coated on the uneven surface of an LSIchip and a hard SiO₂ film coated on the soft film, the hard SiO₂ film onthe surfaces of projecting portions is first polished with the abrasivesof Examples 4 and 5, and the soft film exposed to the projectingportions is polished with the abrasive of Example 6, whereby themountain of each projecting portion gradually lowers and polishing isstopped when the hard SiO₂ film (stopper) in a recessed portion isreached. Thus, the surface of the LSI is planarization.

The present invention makes it possible to produce crystalline cericoxide particles having a particle diameter controlled to any valuewithin the range of 0.005 to 5 μm by blowing oxygen or a gas containingoxygen into a suspension of cerium (III) hydroxide obtained by adding analkaline substance to a cerium (III) salt in an inert gas atmospheresuch as a nitrogen gas at a temperature of 10 to 95° C. and at anatmospheric pressure.

Since crystalline ceric oxide particles can be produced at anatmospheric pressure (normal pressure) by the process of the presentinvention, a bulky high-pressure container is not required, therebyensuring safe operation and low costs.

Since the nucleus generation speed and crystal growth speed ofcrystalline ceric oxide particles can be controlled individually by theconcentration of the cerium (III) salt, the concentration of thealkaline substance, the reaction temperature and the concentration andsupply of the oxidizing aqueous solution in the process of the presentinvention, the particle diameter of the crystalline ceric oxideparticles can be controlled to any value within the range of 0.005 to 5μm.

The ceric oxide according to the present invention is used as anabrasive, ultraviolet absorbing material, catalyst material, fuel cellmaterial and the like. Out of these application fields, it isparticularly useful as an abrasive and ultraviolet absorbing material.

As an abrasive, the ceric oxide of the present invention can be used topolish oxide films and organic films for semiconductor devices, opticalcrystal materials such as glass, quartz crystal and lithium niobate,ceramic materials such as aluminum nitride, alumina, ferrite andzirconia, and metals such as aluminum, copper and tungsten.

As an ultraviolet absorbing material, the crystalline ceric oxideparticles having a particle diameter of 5 to 10 nm of the presentinvention can be used to improve the weatherability of ultravioletabsorbing glass, and ultraviolet absorbing polymer films and plastics.

In the present invention, an abrasive comprising ceric oxide particlesor particles essentially composed of ceric oxide obtained by heatingceric oxide particles or particles essentially composed of ceric oxideobtained by calcining and grinding a composition containing a rare earthelement essentially composed of cerium at a temperature of 50 to 250° C.in an aqueous medium in the presence of an ammonium salt. This abrasiveis composed of surface modified ceric oxide particles and the propertiesof the obtained abrasive composed of ceric oxide particles differdepending on types of a chemical (ammonium salt) used for surfacemodification.

Using an abrasive composed of ceric oxide particles whose surfaces aremodified by heating in an aqueous medium in the presence of an ammoniumsalt having a non-oxidizing anion component, a hard film such as asilicon oxide film (SiO₂ film) of a semiconductor device, inorganicglass, quartz crystal (such as a quartz piece of a quartz crystaloscillator) and a hard substance such as quartz glass can be polished ata higher speed than when using an abrasive composed of ceric oxideparticles of the prior art having the same particle diameter as that ofthe above abrasive.

Using an abrasive composed of ceric oxide particles whose surfaces aremodified by heating in an aqueous medium in the presence of an ammoniumsalt having an oxidizing anion component, a hard film such as a siliconoxide film (SiO₂ film) is polished at a much lower speed than when usingan abrasive composed of ceric oxide particles of the prior art havingthe same particle diameter as that of the above abrasive.

Though polishing for wide application is made possible by using twodifferent abrasives independently, selective polishing is possible inthe field of semiconductor devices by combining the two differentabrasives.

What is claimed is:
 1. An abrasive comprising crystalline ceric oxideparticles for polishing a hard substance selected from the groupconsisting of inorganic glass, quartz glass and quartz crystal, whereinthe crystalline ceric oxide particles have a particle diameter of 0.005to 5 μm and the number of hydroxyl groups on the surface of saidparticles is reduced to no less than one group or increased by oxidizingor reducing the surface of said particles.
 2. The abrasive of claim 1,wherein the hard substance is quartz crystal.
 3. The abrasive of claim1, wherein the hard substance is a quartz piece of a quartz crystaloscillator.
 4. The abrasive of claim 1, wherein the crystalline cericoxide particles are obtained by reacting a cerium (III) salt with analkaline substance in an (OH)/(Ce³⁺) molar ratio of 3 to 30 in anaqueous medium in an inert gas atmosphere to produce a suspension ofcerium (III) hydroxide and immediately blowing oxygen or a gascontaining oxygen into the suspension at a temperature of 10 to 95° C.and at an atmospheric pressure.
 5. The abrasive of claim 4, wherein thegas containing oxygen is air.
 6. The abrasive of claim 1, wherein thecrystalline ceric oxide particles are heated at a temperature of 50 to250° C. in an aqueous medium in the presence of an ammonium salt.
 7. Theabrasive of claim 6, wherein an anion component of the ammonium salt isa non-oxidizing component.
 8. The abrasive of claim 6, wherein theammonium salt having a non-oxidizing anion component is ammoniumcarbonate, ammonium hydrogencarbonate or a mixture thereof.
 9. Theabrasive of claim 6, wherein an anion component of the ammonium salt isan oxidizing component.
 10. The abrasive of claim 6, wherein theammonium salt having an oxidizing anion component is ammonium nitrate,ammonium sulfamate or a mixture thereof.